The present invention relates to an output circuit device suitable for driving a low-impedance load (e.g., a laser diode) at high speed by supplying current thereto.
A circuit used for driving a laser diode for high-speed optical communication of 10 Gbps class converts a high-speed signal, whose rising time and falling time are short, into a large current output, and supplies this current to a laser diode. Therefore, the circuit requires a large-power output-stage transistor, which is large size. To attain high speed, the output stage of this circuit is formed of a differential circuit made up of a pair of transistors, and is connected to a laser diode wherein the collector of a transistor serves as a load. In other words, the circuit is of an open-collector type. The differential circuit of the output stage is driven by supplying a signal from the preceding stage thereto by way of a buffer circuit.
The paired differential transistors constituting the output-stage differential circuit switches from one to the other in response to a variation in base voltage, which is of the order of tens of mV depending upon the input signals. In order to secure a desirable noise margin, the circuit is designed such that the amplitude of an internal signal is of the order of hundreds of mV. At the time of high-speed switching, therefore, a variation in base voltage causes a voltage variation of hundreds of mV at the common emitters of the paired differential transistors of the output-stage differential circuit. When the voltage variation acts on the parasitic capacitor of the transistors of the large-sized output-stage differential circuit, transient current is produced, resulting in unstable operations.
The voltage variation at the common emitters of the paired differential transistors is fed back to the base. However, since the output impedance viewed from the base to the preceding-stage circuit is of a finite value, the voltage variation fed back to the base is amplified by the paired differential transistors once again. This phenomenon is an oscillation phenomenon occurring in a differential circuit and is generally referred to as a ringing phenomenon. The ringing phenomenon adversely affects the waveforms of the output current.
In order to cope with the ringing phenomenon, the conventional art employs an integrator circuit or the like so as to blunt the waveform of the signal supplied to the base of the paired differential transistors of the output-stage differential circuit. In other words, the rising and falling times of the signal are lengthened, and oscillation is prevented thereby. Alternatively, the output impedance of the circuit that precedes the output-stage differential circuit is set to be as low as possible, so as to suppress the ringing phenomenon.
However, the former method inevitably affects the high-speed characteristic, and the original function of the output circuit device (i.e., the function of generating a current output in response to input of a high-speed signal) is degraded. On the other hand, the latter method requires a large amount of current in the circuit, leading to increases in power consumption, the heat generation, etc.